Thermal energy

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  • Created by: emma
  • Created on: 15-05-13 07:59

Specific heat capacity

  • E= MCAT
  • The specific heat capacity, c, of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1K
  • Units of specific heat capacity: j kg-1 K-1 or j kg-1 C-1

The effect that transferred heat has on the temperature of an object depends on:

  • amount of heat energy transferred
  • mass of the object
  • specific heat capacity of material from which object is made
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Measuring the specific heat capacity

  • Heat substance with heater
  • accurate value for c? need a temperature rise of 10K
  • (To improve accuracy start below room temperature and finish above to cancel out gains and losses)
  • E = VIt
  • put data into E= MCAT
  • value for c is probably too big - some energy from the heater is transferred to air and the container.
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Internal energy

Internal energy is the sum of the kinetic and potential energy of the particles within a system

as the temperature of the gas increases,:

  • av. particle speed increases
  • max. particle speed increases
  • the distribution curve becomes more spread out.

energy is constantly transferred between particles in collisions, but the total energy of the system dosen't change. > the average speed of the particles stays the same provided the temperature does.

mean square speed: represents the squared speed of a typical particle.

average Ek is proportional to absolute temperature --> internal energy must also be dependent on absolute temperature.

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Gas laws

Boyle's law:

For a constant mass of gas at a constant temperature, the pressure exerted by the gas is inversely proportional to the volume it occupies.

Charles's law:

For a constant mass of gas at a constant pressure, the volume occupied by the gas is proportional to its absolute temperature

The pressure law:

For a constant mass of gas at a constant volume, the pressure exerted by the gas is proportional to its absolute temperature.

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Ideal gasses

  • If accurate experiments are carries out with a variety of gasses, the gas laws are not perfectly followed.
  •  e.g gas volume cannot be reduced to less than the volume of its molecules, but charles's law suggests that it would be at 0 kelvin.

An ideal gas (one which obeys the three gas laws perfectly) would have the following properties:

  • molecules which have 0 size
  • identical molecules
  • molecules that collide with each other and the walls of their containers without any loss of energy in collisions that take 0 time
  • molecules exert no forces on each othe except during collisions
  • there are enough molecules for statistics to be applied.

The three gas laws can be combined to form the ideal gas equation: pV = NkT or pV=nRT

N = number of molecules of gas, K = boltzmann constant

R= universal gas constant, n = number of moles of gas

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Investigating the gas laws

Boyles's law:

  • length of air in vertical column represents volume of gas
  • pressure measured using barometer
  • pressure against 1/v graph gives straight line > inversley proportional.

The pressure law:

  • data-log measurments of gas pressure and temperature
  • pressure against absolute temperature gives straight line through origin > directly proportional.
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  • Adding thermal energy to an object raises its temperature (as long as it dosen't change state)
  • All molecules in a sample move randomly with a bariety of speeds/ The temperature of a sample is a measure of the average Ek of its molecules.
  • If you took all of the kinetic energy out of a sample, its temperature couldn't fall further. This is absolute 0.
  • If A and B are placed next to each other and energy (called thermal energy/heat) moves from one to the other, A started at a higher temperature.
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